(1) Field of the Invention
The present invention relates to a polyhexamethylene adipamide fiber and a process for the preparation thereof. More particularly, it relates to a polyhexamethylene adipamide fiber having high dimensional stability and fatigue resistance, which is used as a rubber reinforcer for a tire cord, a belt or the like, and a process for the preparation thereof.
(2) Description of the Prior Art
Since a polyhexamethylene adipamide fiber is excellent in tensile strength, toughness, heat resistance, dyeability and colorability, it is broadly used as an industrial material, an interior bedding mateial, a clothing fiber and the like. Especially, since it is excellent in tensile strength, toughness, fatigue resistance and adhesion to rubber, it is widely used as a fiber for tire cords.
Recently, an energy-saving effect is desired even in tire cords and development of tires capable of reducing the fuel consumption in automobiles is required. Accordingly, efforts have been made by tire makers to provide tires having a smaller rolling resistance and a lighter weight. Accordingly, yarns having a higher dimensional stability and a higher tensil strength have been desired for the production of tire cords. Improvement of the durability of tires is necessary not only for attaining an economical effect by prolonging lives of tires but also for improving the safety, and from this viewpoint, yarns having a high fatigue resistance are desired.
A nylon 66 fiber is excellent over a nylon 6 fiber in the heat resistance and dimensional stability and also excellent over a polyethylene terephthalate fiber in the heat resistance, especially the heat resistance under high humidity conditions, and the amine decomposition resistance. However, the nylon 66 fiber is defective in that the fiber is inferior to the polyethylene terephthalate fiber in the dimensional stability. Therefore, in the field of radial carcasses where dimensional stability is required, steel, polyethylene terephtalate and rayon have mainly been used. Since steel and rayon are low in the tensile strength per unit weight, the amount used of cords per tire is increased, resulting in increase of the tire weight and the cost. Polyethylene terephthalate is poor in the heat resistance, especially the heat resistance under high humidity conditions, and therefore, use of polyethylene terephthalate fibers is restricted for truck or bus tires and high-speed tires where the running temperature is high. Under this background, it has been required to improve the dimension stability of a nylon 66 fiber while retaining excellent properties thereof, such as high tensile strength, high heat resistance and high fatigue resistance.
A method for improving the dimensional stability and fatigue resistance of a polyester yarn is disclosed in Japanese Unexamined Patent Publication No. 53-58032. In this method, a polyester composed mainly of polyethylene terephthalate is melt-spun under a high stress and the resulting undrawn filament yarn having a relatively high birefringence of 9.times.10.sup.-3 to 70.times.10.sup.-3 is heat-drawn. As the speed of taking up the undrawn yarn, there is adopted a speed of 1000 to 2000 m/min. After issuance of the above unexamined patent publication, various investigations have been made to improve the dimensional stability and fatigue resistance by drawing high-speed melt-spun yarns. In connection with polyhexamethylene adipamide fibers, Japanese Unexamined Patent Publication No. 58-60012 discloses a method comprising melt-spinning polyhexamethylene adipamide, taking up the spun filament yarn at a speed higher than 2000 m/min and then drawing the filament yarn. However, if the orientation degree of the spun yarn is increased by increasing the spinning speed, the drawability is worsened. This tendency is especially prominent in polyhexamethylene adipamide having a very high crystallization rate. Accordingly, polyhexamethylene adipamide is defective in that the higher the spinning speed, the lower the tensile strength and elongation of the obtained drawn yarn. The inherent function of a tire cord is a reinforcing action, and if the tensile strength and elongation of the tire cord are reduced, it becomes necessary to increase the amount of the yarn used in a tire, resulting in increase of the tire weight and the manufacturing cost.